Resonant dc-converter and method for operating a resonant dc-converter

Abstract

The invention relates to a resonant DC controller (1) and a method for operating a resonant DC controller (1). The DC controller (1) is equipped with a supply input (2) for a supply voltage, a DC voltage output (6) for providing a DC voltage, a pulse generator (3) connected to the supply input (2) on its input side, a resonant converter (4) connected to outputs of the pulse generator (3) on its input side and to the DC voltage output (6) on its output side, an output capacitor (5) connected between a positive terminal (7) and a negative terminal (8) of the DC voltage output (6), and a control unit (9). The pulse generator (3) is configured to generate pulses at a pulse frequency (f) that can be variably specified by the control unit (9) and to output them to the resonant converter (4). The resonance converter (4) has a plurality N of resonance tanks (10, 10-1, 10-2, 10-3, ..., 10-N) with a resonant frequency. According to the invention, the control unit (9) is configured to supply the pulse generator (3) with a first pulse frequency (f1) during an initialization period (19) of the DC controller (1) and to supply the pulse generator (3) with a second pulse frequency (f2) that differs from the first pulse frequency (f1) during a continuous operating period (20) of the DC controller (1) following the initialization period (19). The second pulse frequency (f2) corresponds to an N-fold of the resonant frequency. The method according to the invention describes the DC controller (1) in its operation according to the invention.

Description

Technical area

[0001] The invention relates to a resonant DC controller and a method for operating a resonant DC controller. Technical background

[0002] DC converters are used to generate an adjustable DC voltage from a supply voltage. The supply voltage can be either AC or DC; in the former case, they are often referred to as AC / DC converters, while in the latter case, they are referred to as DC / DC converters.

[0003] The general aim is to achieve the highest possible efficiency and the lowest possible losses during conversion. A DC converter can, for example, be used to charge a battery, whereby the DC voltage generated by the DC converter is adapted to the current battery charge level and thus to the battery's open-circuit voltage, which varies with the charge level. Galvanic isolation is also required for charging traction batteries in electric vehicles, so resonant DC converters with a high-frequency transformer are a well-known class of DC converters suitable for such applications. To emphasize the property of galvanic isolation, the terms explained above are written with slight modifications as AC / DC or DC / DC converters. The DC converters described below can be operated equally well using an AC or DC voltage as the supply voltage.

[0004] The core of a resonant DC converter is a resonant converter, which has at least one resonant tank on the input side, which can be excited to oscillate at a resonant frequency by current or voltage pulses. The resonant frequency is determined by the circuit's dimensions and its parasitic capacitances and inductances. The oscillation generated by the resonant tank is transferred via a primary winding of a high-frequency transformer to its secondary side, where the transferred energy is rectified and buffered in an output capacitor. The voltage across the output capacitor then represents the DC voltage generated by the DC converter. In general, series and parallel circuits of capacitors used as buffer capacitors at the output of a DC converter should be understood as "output capacitors" (singular) in this document, even though in such cases multiple capacitors are involved.

[0005] With resonant DC converters, no energy is stored in the DC converter when switched on, or more precisely in the inductors (resonance tank, transformer, line filter) and capacitors (resonance tank, output capacitor). This results in high inrush currents that can destroy the semiconductor components used. Parasitic inductances also cause oscillation in the DC converter, which can cause high blocking voltages on some of the transistors installed in the DC converter, which can be twice or more the voltages that occur during continuous operation. Accordingly, it is necessary to select transistors with sufficiently high breakdown voltages, which results in higher costs and higher power losses when driving the transistors. All other components must also have a correspondingly higher dielectric strength.

[0006] Against this background, the object of the invention is to provide an improved DC controller and an improved method for operating a DC controller. Summary of the invention

[0007] A first aspect of the invention therefore introduces a DC controller with a supply input for a supply voltage, a DC voltage output for providing a DC voltage, a pulse generator connected on the input side to the supply input, a resonance converter connected on the input side to outputs of the pulse generator and on the output side to the DC voltage output, an output capacitor connected between a positive pole of the DC voltage output and a negative pole of the DC voltage output, and a control unit. The pulse generator is designed to generate pulses at a pulse frequency that can be variably specified by the control unit and to output them to the resonance converter. The resonance converter has a plurality N of resonance tanks with a resonance frequency.According to the invention, the control unit is designed to specify a first pulse frequency to the pulse generator during an initialization period of the DC controller and, during a continuous operation period of the DC controller following the initialization period, to specify a second pulse frequency to the pulse generator that differs from the first pulse frequency. The second pulse frequency corresponds to N times the resonant frequency.

[0008] The invention is based on and incorporates the insight that a resonant converter only achieves good efficiency when operated at the resonant frequency of the resonance tanks. If, on the other hand, the resonant converter is excited with pulses at a pulse frequency deviating from the resonant frequency, the efficiency drops significantly. The inventors have recognized that the reduced efficiency means increasing power loss within the DC converter, which is generally considered to be absolutely to be avoided in the technical field. In contrast, however, the inventors have recognized that by changing the pulse frequency away from the resonant frequency of the resonant converter, the input impedance of the DC converter is increased, so that exceeding the nominal operating current when switching on the DC converter can be avoided or at least significantly reduced.Accordingly, the invention provides for exciting the resonant converter during the initialization period with a pulse frequency different from its resonant frequency. The currents flowing into the resonant converter and the output capacitor can thus be limited to values ​​typical for continuous operation. This not only enables the DC converter to operate stable after power-up in a simple manner, but also avoids additional costs for transistors with higher current carrying capacities.

[0009] Particularly preferably, the first pulse frequency is higher than the second pulse frequency. Although operating the resonant converter below its resonant frequency also results in increased impedance and thus the desired limitation of inrush currents, increased pulse frequencies are preferred during the initialization period because they result in shorter turn-on phases of the transistors, which limits the thermal load on the transistors.

[0010] Preferably, the first pulse frequency and the second pulse frequency differ from each other by at least 30 percent; for example, one pulse frequency can be at least one and a half times, at least twice, or even at least three times the other pulse frequency. The effect of the limitation becomes increasingly greater the further the first pulse frequency is from the second pulse frequency. However, the initialization period should be short because, when the resonant converter is operating away from its resonant frequency, increased power loss occurs in its switching elements, which could otherwise overheat. For this reason, the inrush currents that charge the various inductors and capacitances in the circuit should not be limited too tightly, as this would unnecessarily extend the initialization period.In the best case, the inrush currents and blocking voltages on non-conducting transistors are approximately as large as the currents and voltages occurring at the corresponding point during continuous operation.

[0011] The resonant frequency, for example, can be between 20 and 70 kilohertz. In this frequency range, resonant converters demonstrate good efficiencies while maintaining reasonable costs for the semiconductor components and high-frequency transformer used.

[0012] Preferably, the initialization period lasts less than 120 microseconds. Alternatively or additionally, the initialization period preferably lasts at least 30 microseconds. Within such a period, the inductors and capacitances of the DC converter can be sufficiently charged without the inrush currents and reverse voltages becoming excessively high. However, the initialization period should not be too long, as otherwise the thermal load on the transistors that are not de-energized or de-energized during the initialization period would become excessive (see also above).

[0013] The control unit can additionally be designed to predetermine a time-varying third pulse frequency during a transition period lying between the initialization period and the continuous operation period, which third pulse frequency corresponds to the first pulse frequency at the beginning of the transition period and to the second pulse frequency at the end of the transition period, and in the process to reduce a difference frequency between the third pulse frequency and the second pulse frequency over the transition period. The reduction of the difference frequency can in particular take place continuously or step by step. By not switching the pulse frequency directly from the first to the second pulse frequency between the initialization period and the continuous operation period, improved behavior with regard to control-related aspects of the circuit as well as with regard to electromagnetic radiation and feedback can result.

[0014] In preferred embodiments of the DC converter according to the invention, the pulse generator comprises SiC MOSFET transistors (silicon carbide metal oxide semiconductor field-effect transistors) or GaN transistors (gallium nitride transistors). The pulse generator must satisfy the highest requirements regarding switching frequency and edge steepness of the switched pulses, so the best results are achieved with SiC MOSFETs or GaN transistors.

[0015] The resonant converter, on the other hand, can use more cost-effective IGBTs (Insulated Gate Bipolar Transistors) because the pulses generated by the pulse generator are distributed across the parallel paths of the resonant converter due to the design of the resonant converter with N resonance tanks, so that the switching elements used here are operated at or near the resonant frequency, which means lower switching speed requirements by a factor of N.

[0016] The compromise between fast, but cost-intensive transistors (SiC MOSFETs or GaN transistors) in the pulse generator on the one hand and inexpensive, but slow IGBTs in the resonant converter on the other is therefore particularly advantageous. It is advantageous if the control unit is additionally configured to switch the transistors of the resonant converter, as this ensures that the switching elements of the pulse generator and the resonant converter are switched in a synchronized manner. This makes it possible to switch the switching elements of the resonant converter at times when they are at least approximately free of current and / or voltage, which reduces the power loss in the switching elements and thus results in improved device efficiency.

[0017] Particularly preferably, the pulse generator is designed without an intermediate circuit, i.e., without an intermediate circuit capacitor. Examples of such circuits can be found in International Patent Application WO 2023 / 006949 A1. The invention is particularly suitable for circuits of this type because the elimination of the intermediate circuit capacitor, which represents a relatively large inertia and thus damping in control technology, makes it particularly challenging to reliably transition from the on state to stable continuous operation.

[0018] Particularly preferably, the resonant converter comprises an N-phase high-frequency transformer with N primary windings, each connected to a respective one of the N resonant tanks. The more phases the high-frequency transformer has, the smaller the currents on each of the N paths become, resulting in lower resistive losses. However, the design of the high-frequency transformer becomes more complex with the increasing number of phases, and stray inductances increase, so in practice, high-frequency transformers with two, three, or four phases achieve the best results.

[0019] A second aspect of the invention relates to a method for operating a DC controller having a supply input for a supply voltage, a DC voltage output for providing a DC voltage, a pulse generator connected on the input side to the supply input, a resonance converter connected on the input side to outputs of the pulse generator and on the output side to the DC voltage output, and an output capacitor connected between a positive pole of the DC voltage output and a negative pole of the DC voltage output, wherein the pulse generator is designed to generate pulses with a variable pulse frequency and to output them to the resonance converter, wherein the resonance converter has a plurality N of resonance tanks with a resonance frequency.According to the invention, the pulse generator generates pulses at a first pulse frequency during an initialization period of the DC controller, and pulses at a second pulse frequency different from the first pulse frequency during a continuous operation period of the DC controller following the initialization period. The second pulse frequency corresponds to N times the resonant frequency.

[0020] The method of the second aspect of the invention makes it possible to start up a DC converter of the type described here without allowing the resulting inrush currents to become excessively high. This, in turn, allows the use of cost-effective transistors with lower current-carrying capacity. Furthermore, all voltage-loaded components are relieved, allowing for more cost-effective designs and / or anticipated service life advantages.

[0021] Particularly preferably, the first pulse frequency is greater than the second pulse frequency, which brings with it the advantages described above for the device.

[0022] Preferably, the first pulse frequency and the second pulse frequency differ from each other by at least 30 percent; for example, one pulse frequency can be at least one and a half times, twice, or even three times the other pulse frequency. For the advantages achieved thereby, reference is again made to the explanations regarding the device according to the invention.

[0023] During a transition period between the initialization period and the continuous operation period, the pulse generator can generate pulses with a time-varying third pulse frequency, which corresponds to the first pulse frequency at the beginning of the transition period and to the second pulse frequency at the end of the transition period, while reducing a difference frequency between the third pulse frequency and the second pulse frequency over the transition period. The reduction of the difference frequency can, in particular, occur continuously or stepwise and has the advantages described above for the device. Brief description of the figures

[0024] The invention is described in more detail below with reference to exemplary embodiments. In the following: Fig. 1 a first embodiment of the DC controller according to the invention in a block diagram; Fig. 2 an embodiment of a resonance converter as can be used in the DC controller according to the invention; Fig. 3 a second embodiment of a DC controller according to the invention in a circuit diagram; Fig. 4 a first embodiment of the method according to the invention based on a first diagram in which the pulse frequency f is plotted against time t; Fig. 5 a second embodiment of the method according to the invention based on a second diagram in which the pulse frequency f is plotted against time t; and Fig. 6 a third embodiment of the method according to the invention based on a third diagram in which the pulse frequency f is plotted against time t. Detailed description of the illustrations

[0025] Fig. 1 shows a first embodiment of the DC controller 1 according to the invention in a block diagram. A pulse generator 3 is connected on the input side to a supply input 2, which in the present case is designed merely as an example as a two-pole input for a direct voltage or a single-phase alternating voltage, but could easily also be designed as a multi-phase alternating voltage input. The electrical energy is supplied to the DC controller 1 via the supply input 2, which energy is converted by the DC controller 1 into a direct voltage with a desired voltage value and output via a direct voltage output 6. If a direct voltage is provided at the supply input 2, the DC controller 1 is a DC / DC controller. If, on the other hand, an alternating voltage is provided at the supply input 2, the DC controller 1 is an AC / DC controller.

[0026] Pulse generator 3 is designed to generate pulses according to a frequency specification of a control unit 9 and to output them to a resonance converter 4 connected downstream of pulse generator 3. In the example shown, resonance converter 4 is designed as a three-phase converter (N=3), meaning it has three resonance tanks that receive pulses from pulse generator 3 at staggered times. It is also possible to construct resonance converter 4 as a two-phase converter (N=2) or with more than three phases (N>3).

[0027] The resonant converter 4 can be constructed according to a known design. In this respect, reference is made to the known state of the art. The outputs of the resonant converter 4 are connected to the DC voltage output 6, between whose positive pole 7 and negative pole 8 an output capacitor 5 is connected. The output capacitor 5 serves to filter the DC voltage generated by the resonant converter 4 and to buffer it against possible load fluctuations.

[0028] Fig. 2 shows an embodiment of a resonance converter 4, as can be used in the DC controller 1 according to the invention. In the present case, the resonance converter 4 is designed with N phases, where N is a plurality. Even though the N-phase design is indicated in the figure by four circuit paths, a design with two, three, or more than four phases is also possible. The resonance converter 4 is equipped on the input side with a distribution stage 18, whose task is to distribute the pulses present at the inputs of the resonance converter 4 in turn to downstream resonance tanks 10-1, 10-2, 10-3, ..., 10-N, which all have (at least approximately) the same resonance frequency. The distribution stage 18 is preferably controlled by the control unit 9 of the DC controller 1.

[0029] By exciting the individual resonance tanks 10-1, 10-2, 10-3, ..., 10-N with temporally offset pulses, the resonance tanks 10-1, 10-2, 10-3, ..., 10-N oscillate out of phase with one another. With a three-phase design of the resonance converter 4, the three resonance tanks oscillate offset by 120 degrees; with a four-phase design, by 90 degrees, etc. The distribution stage 18 thus reduces the pulse frequency of the pulses generated by the pulse generator 3 for exciting the resonance tanks 10-1, 10-2, 10-3, ..., 10-N by a factor corresponding to the number N of resonance tanks 10-1, 10-2, 10-3, ..., 10-N or the number N of phases of the resonance converter 4.

[0030] The resonance tanks 10-1, 10-2, 10-3, ..., 10-N are connected to a corresponding number of primary windings 12-1, 12-2, 12-3, ..., 12-N of a high-frequency transformer 11, so that an alternating current flows in the primary windings 12-1, 12-2, 12-3, ..., 12-N at a frequency corresponding to the resonant frequency during resonance operation. For this purpose, the resonance tanks 10-1, 10-2, 10-3, ..., 10-N are connected in parallel with the primary windings 12-1, 12-2, 12-3, ..., 12-N. The resonant frequency is not determined solely by the components of the resonance tank, but is also influenced by the inductance of the high-frequency transformer 11. The current flowing in the primary windings 12-1, 12-2, 12-3, ..., 12-N generates a magnetic field, which in turn induces voltages in the secondary windings 13-1, 13-2, 13-3, ..., 13-N of the high-frequency transformer 11.

[0031] The high-frequency transformer 11 provides galvanic isolation and can also provide voltage transformation, for example, to generate output voltages that are higher than the voltage applied to the supply input. However, in some embodiments of the DC converter 1 according to the invention, the high-frequency transformer can have a winding ratio of 1. Generally, high-frequency transformers with a winding ratio different from that described in Fig. 2 shown different configuration can be used within the scope of the invention. Thus, in Fig. 2 The primary side of the high-frequency transformer 11 is connected in a delta connection, while the secondary side is connected in a star connection. However, a star connection could also be used on the primary side and / or a delta connection on the secondary side. In general, a variety of high-frequency transformer topologies can be used here.

[0032] The voltages induced in the secondary windings 13-1, 13-2, 13-3, ..., 13-N are output to a rectifier 14, which can be designed as an active or passive rectifier. An active rectifier avoids losses that occur in the diodes used in passive rectifiers by switching transistors on and off at appropriate times instead of the diodes. Furthermore, an active rectifier makes it possible to design the DC controller bidirectionally, i.e., to also transport energy from the DC voltage output 6 to the supply input 2. However, active rectifiers require additional control effort. Furthermore, the gate capacitances of the transistors used in this case in turn result in losses that at least partially cancel out the savings achieved by avoiding the conduction losses of the diodes.If an active rectifier is provided, the transistors can also be controlled by the control unit 9, but suitable measures should be taken to maintain galvanic isolation (e.g. optocouplers).

[0033] Fig. 3 shows a second embodiment of a DC converter 1 according to the invention in a circuit diagram. The components belonging to the pulse generator 3, the resonance converter 4 and its main units, distribution stage 18, resonance tanks 10, high-frequency transformer 11 and rectifier 14, are indicated by curly brackets. In the embodiment of the Fig. 3 The supply input (here without reference symbol) is designed for a three-phase AC supply network 15, which is connected to the DC controller 1 via line filter chokes 16. The line filter chokes 16 serve to dampen the effect of the switched DC controller 1 on the supply network and to buffer energy for the current consumption by the downstream stages.

[0034] The pulse generator 3 has a half-bridge for each mains phase of the AC voltage supply network 15, via which the mains phases of the AC voltage supply network 15 are switched through at suitable times to generate voltage pulses. Since no intermediate circuit capacitor is provided in the illustrated embodiment, i.e., the pulse generator is designed without an intermediate circuit, no DC voltage is generated at the interface between the pulse generator 3 and the resonance converter 4. Eliminating the intermediate circuit capacitor, which is widely used in the prior art, reduces the cost of the DC converter 1, but increases the control requirements during operation of the DC converter 1 because the inertia inherent in the control system due to an intermediate circuit capacitor is eliminated.Due to the high demands on the edge steepness of the generated pulses and thus on the switching frequency of the switching elements used in the half-bridges, the half-bridges in the embodiment of the . Fig. 3 designed as SiC MOSFETs.

[0035] A distribution stage 18 of the resonant converter 4 is also constructed from half-bridges, but for the reasons explained above, these half-bridges advantageously feature IGBTs. The distribution stage 18 distributes the voltage pulses generated by the pulse generator 3 to the three resonant tanks 10 of the resonant converter 4, so that a rotating pulse appears at each of the resonant tanks 10.

[0036] Although the pulse generator 3 is equipped with three inputs for the three-phase supply voltage and the resonant converter 4 is equipped with three resonant tanks 10, these numbers do not have to be equal. The number of inputs is generally determined by the number of supply voltage connections, while the number of resonant tanks 10 of the resonant converter 4 is determined by the number of primary windings 12 of the high-frequency transformer 11.

[0037] The basic structure of a resonance converter 4 has already been described using Fig. 2 explained. Fig. 3 shows a possible circuit implementation, whereby in this case the resonance converter 4, in contrast to the example of Fig. 2 was designed as a three-phase system, and the primary side of the high-frequency transformer is connected in a star configuration. The resonance tanks 10 each consist of a capacitor and a coil connected between a respective input of the resonance converter 4 and a star point. The primary windings 12 of the high-frequency transformer 11 are also connected between the inputs and the star point. However, numerous other circuit configurations are also possible, for example, with a delta connection.

[0038] Rectifier 14 is designed as a passive rectifier in this case. However, as already explained, an active rectifier could also be used.

[0039] The output capacitor 5 is embodied as a single capacitor in this case. Different configurations are also possible here, for example, a series connection of capacitors to enable a higher maximum voltage between the positive terminal 7 and the negative terminal 8 than the breakdown voltage of a single capacitor would allow. Likewise, circuits with two capacitors and a center tap between the two capacitors can be used as rectifiers 14, allowing switching between a series and a parallel connection of the two capacitors and thus switching between two operating modes (single current at double voltage or double current at single voltage).

[0040] Fig. 4 shows a first exemplary embodiment of the method according to the invention using a first diagram in which the pulse frequency f is plotted against time t. A switch-on time at time t=0 is immediately followed by an initialization period 19, during which, according to the first exemplary embodiment of the method according to the invention, the pulse generator generates pulses with an (increased) frequency f1. The control of the resonance converter 4 with pulses above its resonance frequency leads to an advantageous reduction in the efficiency of the resonance converter 4 during the initialization period 19, which leads to the limitation of the inrush currents desired according to the invention. Immediately after the expiration of the initialization period 19, a continuous operation period 20 begins, which can last for the entire further operation of the DC controller 1 until it is switched off.During this continuous operating period 20, the resonance converter 4 is excited with pulses of a pulse frequency f 2 suitable for the best possible efficiency, which in the embodiment of the . Fig. 4 lower than the pulse frequency f 2.

[0041] Fig. 5 shows a second embodiment of the method according to the invention using a second diagram in which the pulse frequency f is plotted against time t. In contrast to the embodiment of Fig. 4 Here, after the end of the initialization period 19, the pulse frequency f is continuously approximated to the pulse frequency f 2 for the continuous operation of the DC controller 1 over a transition period 21, which has the advantages explained above.

[0042] Fig. 6shows a third exemplary embodiment of the method according to the invention using a third diagram in which the pulse frequency f is plotted against time t. A transition period 21 is also provided in the third exemplary embodiment of the method according to the invention, although here the transition from the initial pulse frequency f 1 to the pulse frequency f 2 for continuous operation takes place in several steps. This can be implemented more easily in terms of control and circuitry than as shown in the previous exemplary embodiment.

[0043] The invention has been explained in more detail with reference to illustrations of exemplary embodiments. However, the invention is not intended to be limited by the exemplary embodiments, but is defined solely by the following patent claims. List of reference symbols

[0044] 1DC controller 2Supply input 3Pulse generator 4Resonance converter 5Output capacitor 6DC voltage output 7Positive pole 8Negative pole 9Control unit 10, 10-1, 10-2, 10-3, ..., 10-NResonance tank 11High-frequency transformer 12, 12-1, 12-2, 12-3, ..., 12-NPrimary winding 13, 13-1, 13-2, 13-3, ..., 13-NSecondary winding 14Rectifier 15Supply network 16Line filter choke 18Distribution stage 19Initialization period 20Continuous operation period 21Transition period

Claims

1. A DC controller (1) with a supply input (2) for a supply voltage, a DC voltage output (6) for providing a DC voltage, a pulse generator (3) connected on the input side to the supply input (2), a resonance converter (4) connected on the input side to outputs of the pulse generator (3) and on the output side to the DC voltage output (6), an output capacitor (5) connected between a positive pole (7) of the DC voltage output (6) and a negative pole (8) of the DC voltage output (6), and a control unit (9), wherein the pulse generator (3) is designed to generate pulses with a pulse frequency (f) that can be variably predetermined by the control unit (9) and to output them to the resonance converter (4), wherein the resonance converter (4) has a plurality N of resonance tanks (10, 10-1, 10-2, 10-3, ..., 10-N) with a resonance frequency, characterized by the fact thatthe control unit (9) is designed to predetermine a first pulse frequency (f1) to the pulse generator (3) during an initialization period (19) of the DC controller (1) and to predetermine a second pulse frequency (f2) to the pulse generator (3) that is different from the first pulse frequency (f1) during a continuous operation period (20) of the DC controller (1) following the initialization period (19), the second pulse frequency (f2) corresponding to N times the resonance frequency.

2. The DC controller (1) of the preceding claim, wherein the first pulse frequency (f1) is greater than the second pulse frequency (f2).

3. The DC controller (1) of any one of the preceding claims, wherein the first pulse frequency (f2) and the second pulse frequency (f2) differ from each other by at least 30 percent.

4. The DC controller (1) of any one of the preceding claims, wherein the resonance frequency is between 20 and 70 kilohertz.

5. The DC controller (1) of any one of the preceding claims, wherein the initialization period (19) lasts less than 120 microseconds.

6. The DC controller (1) of any one of the preceding claims, wherein the initialization period (19) lasts at least 30 microseconds.

7. The DC controller (1) of one of the preceding claims, wherein the control unit (9) is designed to predetermine a time-varying third pulse frequency during a transition period (21) lying between the initialization period (19) and the continuous operation period (20), which third pulse frequency corresponds to the first pulse frequency (f1) at the beginning of the transition period (21) and to the second pulse frequency (f2) at the end of the transition period (21), and in the process to reduce a difference frequency between the third pulse frequency and the second pulse frequency (f2) over the transition period (21).

8. The DC controller (1) of one of the preceding claims, wherein the pulse generator (3) comprises SiC MOSFET transistors or GaN transistors.

9. The DC controller (1) of any one of the preceding claims, wherein the resonant converter (4) comprises IGBTs.

10. The DC controller (1) of one of the preceding claims, wherein the pulse generator (3) is designed without an intermediate circuit.

11. The DC controller (1) of any one of the preceding claims, wherein the resonant converter (4) comprises an N-phase high-frequency transformer (11) having N primary windings (12, 12-1, 12-2, 12-3, ..., 12-N), each connected to a respective one of the N resonant tanks (10, 10-1, 10-2, 10-3, ..., 10-N).

12. A method for operating a DC converter (1) having a supply input (2) for a supply voltage, a DC voltage output (6) for providing a DC voltage, a pulse generator (3) connected on the input side to the supply input (2), a resonance converter (4) connected on the input side to outputs of the pulse generator (3) and on the output side to the DC voltage output (6), and an output capacitor (5) connected between a positive pole (7) of the DC voltage output (6) and a negative pole (8) of the DC voltage output (6), wherein the pulse generator (3) is designed to generate pulses with a variable pulse frequency and to output them to the resonance converter (4), wherein the resonance converter (4) has a plurality N of resonance tanks (10, 10-1, 10-2, 10-3, ..., 10-N) with a resonance frequency, characterized by the fact thatthe pulse generator (3) generates pulses with a first pulse frequency (f1) during an initialization period (19) of the DC controller (1) and generates pulses with a second pulse frequency (f2) different from the first pulse frequency (f1) during a continuous operation period (20) of the DC controller (1) following the initialization period (19), wherein the second pulse frequency (f2) corresponds to N times the resonance frequency.

13. The method of the preceding claim, wherein the first pulse frequency (f1) is greater than the second pulse frequency (f2).

14. The method of any one of claims 12 or 13, wherein the first pulse frequency (f2) and the second pulse frequency (f2) differ from each other by at least 30 percent.

15. The method of one of claims 12 to 14, wherein the pulse generator (3) generates pulses with a time-varying third pulse frequency during a transition period (21) lying between the initialization period (19) and the continuous operation period (20), which third pulse frequency corresponds to the first pulse frequency (f1) at the beginning of the transition period (21) and to the second pulse frequency (f2) at the end of the transition period (21), and in the process reduces a difference frequency between the third pulse frequency and the second pulse frequency (f2) over the transition period (21).

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